How to symmetrize shape keys?
If you want to mirror some shape keys without doubling the quantity of movement for the center line of vertices, you need to use at least one weightgroup with all vertex at 1 except for the middle ones which will be at 0.5.



Individual shape will have their movement on center line halved, but it's a common practice. Here I show one weightgroup (1-0.5-1) but you will often see two weightgroups (1-0.5-0) and (0-0.5-1).

You can watch this video tutorial from Angela Guenette for this process, and I highly invite you to watch the whole series.